The invention relates to a current amplifier primarily intended for use in integrated circuits. In one application, the current amplifier has proven useful in photo current handling circuitry. In photo sensor applications it has been found that silicon integrated circuit (IC) fabrication readily admits of including photodiodes directly on the IC chip. Silicon photo response is readily adaptable to photographic applications and is extremely linear over a very wide range of light levels. Reasonable sized photo dectectors can respond linearly over the six order of magnitude of light ranging from sunlight to dim room lighting. However, the low light levels may result in picoampere photo currents which require special biasing and control circuitry. U.S. Pat. No. 4,118,621 which was issued Oct. 3, 1978 to Dennis M. Monticelli and Robert S. Sleeth, relates to a Photo Electrically Biased Photodiode Operational Amplifier. Here, two photodiodes having ratioed areas are incorporated into a single IC. The sensor photodiode is zero biased at low light levels to reduce leakage currents that, if uncontrolled, could mask the light-related current. The current amplifier used, although offering high gain, high linearity, and wide dynamic range, requires a third terminal for the power supply.
In the prior art FIG. 1), the photo current sensor is coupled to a single transistor for current amplification. This circuit configuration yields two-terminal operation, a desirable feature. Unfortunately, it also suffers from numerous disadvantages, inasmuch as the current multiplication factor is equal to the DC current gain (Beta) of the transistor. The current multiplication factor is loosely controlled in fabrication resulting in a wide variation (nominally 3:1) in current gain. The current multiplication factor is also dependent upon temperature, voltage bias, and current level. The latter manifests as non-linearity of output vs input for a wide range of input current levels.